Mental illnesses are often classified according to their severity, which is estimated in terms of:

• the distress the symptoms of the illness cause
• the impact the symptoms have on the individual's behaviour
• whether the symptoms affect the day to day functioning of the individual
• whether the symptoms also have broader effects on the family and society.

The majority of mental health disorders cause some level of distress to the individual concerned, but they have limited broader effects on the person's day-to-day life, work, family or society. But about 5% of the population (1 in 20 individuals) have conditions that affect or interfere with their life seriously. Of those with a mental illness, almost one-third (3 in 10) are severely affected — in other words they have an SMI.